1. Field of the Invention
The present invention relates to a process for soldering a member to another member. More specifically, the present invention relates to a process for soldering a land formed on a substrate to an electrode (e.g. a lead) of an electronic component in the production of an electronic circuit board. Furthermore, the present invention also relates to a connecting structure, more specifically an electronic circuit board, produced by such process for soldering.
2. Description of Related Art
in the production of an electronic circuit board to be used for an electronic device and so on, a reflow soldering process is known as a process for mounting an electronic component on a substrate, more specifically, for physically connecting a lead taken out from the electronic component, to a land formed on the substrate.
In a general reflow soldering process, a so-called cream solder is applied by screen printing onto a land that is a part of a patterned wiring formed on a substrate. The cream solder is usually a mixture of soldering powder of a solder material, and a flux including rosin, an activator and a solvent. After that, an electronic component is located on a predetermined portion of the substrate so that a lead taken out from the electronic component adheres to the cream solder applied on the land. The thus-obtained substrate, on which the electronic component is located through the cream solder, is heated at a temperature of at least the melting point of the used solder material so as to activate the flux and melt solder material of the soldering powder in the cream solder, and to evaporate (or volatilize) off other components such as the flux in the cream solder. Thereafter, the resultant product is cooled (or subjected to radiational cooling) such that the molten solder material is solidified. The solidified solder material forms a connecting part between the lead of the electronic component and the land of the substrate to electrically and physically connect them to each other. Although other component(s) such as the flux other than the solder material may be present in the connecting part, such other component is excluded by phase separation form the solder material during the heating. Therefore such component is not present inside the connecting part and only remains in a small amount on the surface of the connecting part. Thereby, an electronic circuit board is obtained, wherein the electronic component is mounted on the substrate by the connecting part (or the soldering part) which is substantially composed of the solder material.
As the solder material, an Sn—Pb based material, in particular an Sn—Pb based material having an eutectic composition (hereinafter simply referred to as an Sn—Pb eutectic material also) is generally used. The eutectic composition of the Sn—Pb based material is an Sn-37Pb composition (i.e. a composition consisting of 37% by weight of Pb and the rest (63% by weight) of Sn). It is known that the Sn—Pb based material has a melting point of 183° C. in the case of this eutectic composition.
Recently, a manner of disposing an electronic device including the electronic circuit board described above becomes an issue, and this causes a concern about influence on the global environment or human bodies due to lead (Pb) contained in the conventional solder material. For that reason, there is a movement for using a solder material containing no lead, namely a lead-free solder material, in place of the conventionally used Sn—Pb based material, and efforts have been made to put the lead-free solder material into practical use.
At present, materials having various compositions have been suggested as the lead-free solder material. One of them is an Sn—Bi based material. As a result of recent studies, the eutectic composition of the Sn—Bi based material is roughly an Sn-58Bi composition (i.e. a composition consisting of 58% by weight of Bi and the rest (42% by weight) of Sn). It has turned out that the Sn—Bi based material has a melting point of 139° C. in the case of this eutectic composition.
Considering the thermal resistance of the electronic component to be mounted on the substrate as well as the guaranteed temperature range of a product provided with the electronic circuit board, the lead-free solder material desirably has a melting point that is sufficiently low so as not to damage the electronic component, and is higher than the guaranteed temperature range for thermal resistance of the product. The melting point of the Sn—Bi based material as above is lower than the melting point of other lead-free solder material such as an Sn—Ag based material and even that of the conventional Sn—Pb based material, and higher than the guaranteed upper limit temperature for thermal resistance of a product. Therefore, the Sn—Bi based material has an advantage in that it becomes possible to conduct soldering at a low temperature. Accordingly, the Sn—Bi based material can be regarded as a strong candidate for a substitute for the Sn—Pb based material.
The eutectic composition of an Sn—Ag based material, which has similarly been suggested as the lead-free solder material, is a roughly Sn-3.5Ag composition (i.e., a composition consisting of 3.5% by weight of Ag and the rest (96.5% by weight) of Sn). It has also turned out that the Sn—Ag based material has a melting point of 221° C. in the case of this eutectic composition. From the viewpoints of the thermal resistance of the electronic component to be mounted on the substrate, and of the adaptability for the existing soldering process, the lead-free solder material desirably has a melting point which is sufficiently low so as not to damage the electronic component and which is relatively close to the melting point of the conventional Sn—Pb based material. However, since the Sn—Ag based material has a melting point higher than that of the conventional Sn—Pb based material and therefore it is difficult to adopt the Sn—Ag based material for the existing soldering process as it is.
However, it has also turned out that, by adding Bi to the Sn—Ag based material to prepare an Sn—Ag—Bi based material, the melting point thereof is made lower and closer to the melting point of the conventional Sn—Pb based material. For example, the Sn—Ag—Bi based material has a melting point of 210° C. in the case of an Sn-3Ag-3Bi-3In composition (i.e. a composition consisting of 3% by weight of Ag, 3% by weight of Bi, 3% by weight of In and the rest (91% by weight) of Sn). For this reason, the Sn—Ag—Bi based material can also be regarded as another strong candidate for a substitute for the Sn—Pb based material.
By using the Sn—Bi based material or the Sn—Ag—Bi based material in place of the Sn—Pb based material, there is an advantage in that soldering of an electronic component to a circuit board can be conducted while avoiding thermal damage to the electronic component. However, from a result of a test for continuous use of the electronic circuit board obtained thereby under a high temperature condition, the inventors found that a connecting part between the land of the substrate and the lead of the electronic component degraded and a sufficiently high thermal fatigue strength could not be obtained.
It is considered that such degradation (or deterioration) results from following mechanism. Tin (Sn) contained in the Sn—Bi based material or the Sn—Ag—Bi based material contacts copper (Cu) used for a material of the land and the lead. Then, an intermetallic compound of Sn—Cu is formed at (or in the vicinity of) interfaces between the connecting part and the land and between the connecting part and the lead. As a result, the concentration of bismuth (Bi) at the interfaces becomes relatively high.
More detail explanation hereinafter will be made with reference to FIG. 5, as to an electronic circuit board 80 which is produced by soldering an electronic component to a substrate while using a cream solder containing soldering powder of the Sn—Bi based material in place of the Sn—Pb material in the conventional reflow soldering process as described above.
In this electronic circuit board 80, a lead 69 taken out from an electronic component 67 is electrically and mechanically connected to a land 63 formed on a substrate 61 through a connecting part 65. The land 63 is generally made of Cu and is formed integrally with a patterned wiring. The lead 69 is generally composed of a base material 69a of Cu and a plating 69b of an Sn—Pb eutectic material which coats the base material 69a. The connecting part 65 results from the cream solder by a heat treatment, and it is substantially made of a solder material derived from the soldering powder as mentioned above.
During the heat treatment, Cu consisting in the land 63 diffuses into the solder material to be combined with Sn since the solder material is directly contacted with the land 63. As a result, an intermetallic compound 73 of Sn—Cu is formed at (or in the vicinity of) an interface of the contact between the connecting part 65 and the land 63.
Further, during the heat treatment, the solder material of the soldering powder melts, and the plating 69b of the Sn—Pb eutectic material also melt since the melting point of the Sn—Pb eutectic material is generally lower than the temperature of the heat treatment. A part of the plating 69b which part is contacted with the molten solder material, melts and diffuses into the solder material. For this reason, the plating 69b of the lead 69 is partially removed off, so that the base material 69a comes to be in direct contact with the molten solder material. Therefore, Cu consisting in the base material 69a of the lead 69 diffuses into the solder material to be combined with Sn similarly to the above. An intermetallic compound 71 of Sn—Cu is formed at an interface of the contact between the connecting part 65 and the lead 69.
Since the intermetallic compound 71 of Sn—Cu is formed at the interfaces in this manner, the concentration of Bi relatively increases at local regions surrounding the intermetallic compound which exists in the interfaces. As a result, a phase 75 in which Bi is present at a relatively high concentration is formed (hereinafter which phase simply referred to as a concentrated Bi phase). Particularly in the case where the substrate to which the electronic component has been soldered is located under a high temperature for a long period, more amount of Cu diffuses into the connecting part (or the solder material), so that the formation of the intermetallic compound of Sn—Cu is proceeded, and the concentrated Bi phase 75 is consequently enlarged (or grown). It is considered that the concentrated Bi phase 75 causes degradation of the connecting part since the concentrated Bi phase 75 is so weak.
Especially in a case where the electronic component is plated with the Sn—Pb material such as the Sn—Pb eutectic material, the degradation of the connecting part was remarkably caused. It is considered that such degradation results from the following mechanism. Once the concentrated Bi phase 75 is formed at the interfaces between the connecting part 65 and the lead 69 and/or between the connecting part 65 and the land 63, an alloy of Sn—Bi—Pb is likely to be formed since lead (Pb) contained in the plating material for the lead is readily combined with bismuth (Bi). Such Sn—Bi—Pb alloy has a low melting point of about 98° C. In the case where the substrate is placed under a high temperature for a long period, the Sn—Bi—Pb alloy formed in the connecting part melts. Once the molten Sn—Bi—Pb alloy is formed, growth of the alloy phase is promoted. As a result, it is considered that the degradation of the connecting part has occurred.
At present, the movement away form lead (Pb) has also been advanced as to a plating material of a lead of an electronic component. In the current situation during a transitional period, however, there is a case where the Sn—Pb based material is still used. Such a case cannot be disregarded, either.
Additionally, an intermetallic compound of Sn—Cu may be formed at interfaces between the connecting part and the land and/or between the connecting part and the lead also in a conventional case as in the prior art where the cream solder containing soldering powder of the Sn—Pb based material is used, since copper (Cu) as a material of the land and/or the lead diffuses into the connecting part and combines with tin (Sn) contained in the Sn—Pb based material. However, the intermetallic compound of Sn—Cu is stable. Furthermore, Bi is not present at the connecting part in such case. Therefore, even though the intermetallic compound of Sn—Cu is formed, a concentrated Bi phase is not formed and thus an Sn—Bi—Pb alloy is not formed. It is considered that the problem of the degradation in the connecting part did not occur in such conventional case.
There is a case where the base material 69a of the lead 69 is not made of Cu, but made of another material such as an Fe-42Ni alloy material (i.e. an alloy material consisting of 42% by weight of Ni and the rest (58% by weight) of Fe). In this case, even if the plating 69b of the lead 69 melts and diffuses into the solder material to contact with the base material 69a directly, the intermetallic compound 71 of Sn—Cu is not formed at the interface of the lead 69 and the connecting part 65. On the other hand, the intermetallic compound 73 of Sn—Cu is again formed at the interface of the land 63 of Cu and the connecting part 65, so that it brings about the enlargement of the concentrated Bi phase and, in cases where the plating material contains Pb, the formation of an Sn—Bi—Pb alloy. Thus, when the resultant electronic circuit board is subjected to a test for continuous use under a high temperature condition, the degradation of the connecting part occurs. It is not possible also in this case to avoid the problem of an insufficient thermal fatigue strength.
The problems mentioned above arise by subjecting the connecting part to a heat treatment for a long period. Therefore, problems similar to those that arise in the test mentioned above would also arise in a case where a heat treatment is performed twice or more, for example, in a production of a so-called “double sided-reflow soldered substrate” or a so-called “double sided-flow soldered and reflow soldered substrate” as the electronic circuit board. The former substrate is produced by conducting a reflow soldering on one side of a substrate to connect an electronic component to the substrate, and by conducting a reflow soldering on the other side of the substrate to connect an electronic component to the substrate. The latter substrate is produced by conducting a flow soldering on one side of a substrate to connect an electronic component to the substrate, and by conducting a reflow soldering on the other side of the substrate to connect an electronic component to the substrate.
As described above, the cases of using the Sn—Bi based material were explained, and similar problems may also occur on using other solder materials at least containing Sn and Bi, such as the Sn—Ag—Bi based material.